It is known in the art of automatic process control, wherein machines are employed to sequentially carry out the steps of a process, to alternately enable and disable machines respectively associated with each step of the process in accordance with a predetermined program. It may be desirable to control the order of performance of the process steps and the duration of each step as a function of absolute time as measured by a clock or in accordance with the instantaneous position of a movable part of a machine employed in the process. It is further known in the art to utilize mechanical cams to actuate switches for enabling and disabling the machinery associated with the various steps of the process which cams move in response to the movement of a clock or the movement of the machine part upon which enabling of the process steps depends.
It is also known in the art of process control to utilize means in which electronic signals may be stored according to a predetermined program for governing the sequence and duration of the process steps. In such devices electronic switches are programmed to enable and disable the machinery associated with the respective process steps according to a predetermined sequence. The start and stop points for each step and the duration of each step are set by manually actuating switches associated with various points in the process cycle or by utilizing a keyboard electrically connected to a memory to establish switching points at which the machinery for accomplishing the various steps of the process is to be enabled and disabled. An example of the manually set process control apparatus is the solid-state limit programmer series SLP/SNA manufactured by Sequential Information Systems, Inc. of Elmsford, New York. An example of the keyboard type of process control apparatus is the Model M1000 Programmable Limit Switch manufactured by National Controls Corporation of Addison, Illinois.
Both of the aforementioned programmable devices are disadvantageous in that the process control program must be manually entered each time the process is to be run by manually setting each point in the process control cycle. Where the process control machinery is to repeat a previous process different from the process last executed by the machine the program must again be manually entered. Manual entry of a program step by step is time-consuming and provides an opportunity for errors in programming the control apparatus. It is therefore desirable to provide a process control apparatus for which a program may be facilely recorded and the record of the program used diretly to program the control apparatus. It is further desirable that the programming of the record for use in process control require little skill and present to the operator a readily visible pattern representative of the process sequence.
A particularly desirable format for recording a process program involves the use of a program card having parallel ruled lines with the length of an area defined by adjacent lines being an analog of the total range of the meausrement of which process control is a function. The measurement analog is subdivided into segments which are shaded to indicate one state of an output process enabling switch used to enable and disable a machine associated with one or more process steps with the remaining unshaded segments being analogous to values of the process control measurement during which the output switch is to occupy the other of its enabling and disabling states. A number of adjacent program areas may be provided on a single program card with each area associated with a separate respective output switch used during the process cycle.
The National Controls Corporation Model M1000 Programmable Limit Switch employs a card on which a program is drawn by shading segments of a ruled strip. The card is placed face up on the programming portion of the device adjacent the keyboard so that the programmer may keypunch the program into the apparatus while viewing the card. Since the program card must be read by the programmer and the program then keypunched by the programmer to insert the information on the card into the process controller memory it is possible that the keypunched program may differ from the program shown on the card due to keypunching errors and/or misreading of the card by the programmer.